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Earthing systems and other things

  1. Jan 25, 2014 #1
    I recently started working at ABB as a engineer. I wish someone here could help with some questions:

    1) In a single phase IT system, why does the circuit breakers have to be bipolar? I believe it is because of a second fault occurence, but i don't really understand what happens.

    2) If you connect a switchgear to the same earthing system as the transformer neutral is connected, instead of having a cable to directly perform this connection, does it makes your system a TT system? Or the fact that the switchgear is in the same earthing system as the source neutral makes it a TN system?

    3) In a three phase system whose neutral is grounded through a resistor, why is it not recommended the existence of single phase loads?

    4) Is it necessary to place a CB before and after a potential transformer to guarantee its protection? Does these CB need to have any special features?

    If anyone knows any articles or papers about these subjects I'd appreciate!
     
  2. jcsd
  3. Jan 25, 2014 #2

    Baluncore

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  4. Jan 26, 2014 #3

    berkeman

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    Great reply by Baluncore :smile:

    I do have to ask -- are they not giving you any training or mentoring at your new job? Why do you have to ask these on-the-job questions on the Internet?
     
  5. Jan 26, 2014 #4
    As surprising as it seems, many things are done in projects without anyone knowing why they're done.
     
  6. Jan 26, 2014 #5

    FOIWATER

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    Unfortunately not too surprising. Even for a company as big as
     
  7. Jan 26, 2014 #6
    As for Q1 -- I am not sure what is meant by "IT", and I am assuming by "bipolar" you mean the circuit breaker has 2 contacts, therefore these are connected to two phases (energized legs) & NOT one Phase Leg and Neutral....? A diagram would help clarify your questions. If the load is connected to two energized phases - you need to disconnect both phases from the load - even though this uses two phases it is still referred to as a single phase load.

    2) Earthing is its own specialty but you need to know this in this fields, even if for your own personal safety, but the point of earthing being that all parts of the system that are earthed should be safe to touch ( zero potential) at all times including during faults and mis-operaitons, anywhere that current flows during a fault can result in lethal voltage, so each location should have it's own conductor to the earthing point, to reduce the possibility of voltage at that point. ( Earthing is not about load current)

    3) Because the phase imbalance (current from the single phase load) causes current in the neutral resistor. This is not a case IMO of "not recommended" -- this should not be allowed.

    4) Potential (voltage) Transformer ? or Power Transformer? Ether way -- CBs are about defining zones of protection. The CB on the Line side transformer - is primarily there to "protect" the transformer, and the Load side to protect the downstream "load" devices ( cables, bus, additional transformers etc.) If the real load is too much for the cables to supply - the LOAD side breaker should trip. The LOAD side breaker will also protect the transformer from overload, but the "problem " is still the load. The Load side breaker in these cases should always trip before the Line side breaker(or fuses) - this is called coordination of protection.

    Are you in field service? I can then understand the people often not knowing the why ( but someone should be able to answer these questions for you) - if you are working in an engineering office - then you are not talking to the right people ( keep asking - do not accept "that is how it is done"). Lastly - physically working on these devices ( touching them) you must know these things - you can not THINK something is safe you have to KNOW - and if you do not understand these concepts - then it is impossible for you to KNOW you are safe..

    I was an FSE for ABB In the US for 8 years ( 3 years before that with a private testing co.) - where are you located?
     
  8. Jan 26, 2014 #7
    Answers in red!
     
  9. Jan 26, 2014 #8
    Sorry I should have read the Wiki including thew IEC terminology. In the WIki - there is not a single phase ( Line+ Neutral) configuration. However assuming a Line and Neutral - and there is no bond of the neutral to Earth - in a fault (technically at any operating point) the Neutral can have high voltage present. So to isolate the load the neutral needs to be disconnected.

    As for point 2 - If the switchgear has an independent connection to earth it is TT - as you have described it is a TN-S ( per the wiki) - this is not just about if a protective device does not operate, but when the system in in a fault condition - it's actual state may not be known. The local - independent earth is safest - if you are standing on the earth - and you touch the switchgear - is the circuit you compete - local or does it have to run all the way back to the transformers neutral - earth bond point. SO IMO the TN is much more hazardous than TT.

    3) -- When you allow single phase loads you should never assume the loads are balanced ( there are a few exceptions - like dedicated electric heat - but this is still poor practice). If the loads are not balanced there will be current in the neutral. The high impedance bonded neutral ( here in the USA) is typically for 3 phase load(no neutral connected to the load), and the neutral is used to detect ground faults without the need to shutdown ( reduce interruptions) - or when the transformer is paralleled (for phase angle matching - etc) with others and the ground fault current is limited. This case usually has an alarm for low amounts of neutral current --- not a trip.

    IN the specific case - High Impedance grounded neutrals were rare and used for very specific purposes and this requires rethinking of a number of elements ( all of them) in the system. In one case a customer was being fed with high impedance grounded transformer by the utility, however the customer switchgear was not designed for this...almost all of the ratings were sufficient except the Surge / Lightening Arrestors....in a grounded system they will only see L-N voltage during a ground fault of a phase..... however in a high impedance grounded system, they can see L-L voltage.... so we had a lot of damage - all over the place.
     
  10. Jan 26, 2014 #9

    AlephZero

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    Sometimes, things are done a certain way because that is what the legal requirements or codes of practice in the industry say. There may be other ways that are just as safe, cost effective, etc, but if you want to sell your products, you follow the rules. (You can also work to get the rules changed, but that's a different issue from selling stuff).

    But I'm with Berkeman here: somebody in your company will know about these questions, and quite likely there will be some internal reports about it, and/or a company web site or wikipedia system that has the answers.

    But if you are a new starter, you can't learn everything at once (and you might be in shock from discovering with how much there is to learn that wasn't covered in your university courses!), so answering these type of questions might not be the top priority in the short term so far as the company is concerned. The first priority is probably getting you productive, and not getting other people less productive while they answer all your general questions!
     
  11. Jan 26, 2014 #10
    In a IT earthing there is no active or neutral conductors, just line 1, line2 etc. So all conductors need to be switched. In the occurrence of a second earth fault the system will behave similar to TN or TT system;If all exposed conductive parts of the installation are interconnected by a protective earth then a second fault would result in a phase to phase or phase to neutral short circuit, resulting in one or both of the protective devices feeding the faulted circuit tripping. Ideally you only want one to trip. If the exposed conductive parts are connected to an independent earth electrode and protected by an RCD then it will behave similar to a TT system during the second fault.

    You need to clear the second fault because you will now have two voltages of a different polarity present on equipment frames. If two people are touching the faulted equipment or one person simultaneously touches the two faulty equipment frames they receive a shock and could be electrocuted.

    In a TN system all exposed conductive parts are connected to the neutral at some point. Either at the source only(TN-S), each installation(TN-C-S) or at the equipment(TN-C). A TT system means that all exposed conductive parts are connected to a earth electrode, in which case all circuits need to be protected by an RCD.

    I've always wondered this but have never been able to find an actual answer. My reasoning would is that the neutral conductor can and is usually sized much smaller than the phase conductors. This would mean if a second fault occurs in an IT system and one of the faulty conductors is the neutral, there could be a scenario where not enough fault current is created to trip either protective device. Again I'm not sure on this one.

    Hope this helps.
     
  12. Jan 26, 2014 #11
    Thank you all for your answers! I'll read them carefully and I'll ask again if I need more help!
     
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